computer doesnt turns on for 1 second then turns off help!!!

[spider monkey m]

okay i just put together my pc and whenever i press the start button it just turns on for 1 second and the fans spin then if i press the power button again it doesnt work so i have to turn the power supply switch off then back on then it does the same and sometimes when i press the power button it doesnt turn on at all i have checked all the plugs are in correctly and sometimes the power led is on and sometimes it isnt im sure my power supply works becuase led on motherboard lights up,a couple of days ago it would work but i couldnt get any display from my video card then it slowly started to not work like one day the computer will start up for that 1 sec and then later on it just wont start up
can anyone please help me?